Process for separating solids from aqueous 1,4-butynediol solutions

ABSTRACT

Solids are separated from aqueous 1,4-butynediol solutions by passing a solids-containing aqueous butynediol solution in the downflow mode through a column and thus bringing it into contact with a solvent which has a lower density than the solids-containing butynediol solution and forms a second phase with the latter, with the solvent rising in countercurrent to the aqueous butynediol solution, the solid accumulating at the interface between the aqueous butynediol and the solvent and the solid being removed from the column by taking off a mixture of aqueous butynediol and solvent.

The present invention relates to a process for separating solids fromaqueous 1,4-butynediol solutions.

1,4-Butynediol is prepared on an industrial scale from acetylene andaqueous formaldehyde solution over copper catalysts (Weissmel, Arpe,Industrielle Organische Chemie, 2nd Edition, Verlag Chemie, p. 94).Butynediol is an important intermediate from which 1,4-butanediol can beprepared by hydrogenation, this being used as an alcohol component inpolyesters and being able to be converted into tetrahydrofuran by ringclosure.

Butynediol is obtained industrially as an aqueous solution containingsmall amounts of solids. During the preparation of butynediol, secondaryreactions between CO₂ and formaldehyde or other intermediates in thereaction lead to the formation of red polymers which are insoluble inaqueous butynediol solution.

The polymeric solid coats all parts of the plant such as tube walls andreactor walls with a film which can be removed mechanically only withdifficulty. On the other hand, if the polymeric solid remains suspended,sedimentation is uneconomical in view of the low concentration, sincesubsequent process steps over highly selective catalysts rule out theaddition of sedimentation aids, because these have an adverse effect onthe selectivity of the catalysts.

Furthermore, the butynediol solution contains solid particles which areattributable to abrasion of the catalyst. Depending on the reactionconditions in the preparation of butynediol, the proportion of abradedcatalyst material is generally from 20 to 80% by weight of the solid inthe aqueous solution. In general, the concentration of the solid in theaqueous butynediol solution is from about 1 to 1000 ppm. The abradedcatalyst material leads to mechanical problems, particularly in pumps.

Furthermore, said solids deposit on the catalyst for the hydrogenationto butene or butanediol which generally follows the preparation ofbutynediol. The solid can be removed only by time-consuming andexpensive stopping of plants and mechanical cleaning of affected plantparts.

It is an object of the present invention to provide a process whichallows economical separation of solids from aqueous 1,4-butynediolsolutions. The process should be able to be used on an industrial scale.In particular, it should be suitable for removing low concentrations ofsolids.

We have found that this object is achieved by a process for separatingsolids from aqueous 1,4-butynediol solutions, which comprises passing asolids-containing aqueous butynediol solution in the downflow modethrough a column and thus bringing it into contact with a solvent whichhas a lower density than the solids-containing butynediol solution andforms a second phase with the latter, with the solvent rising incountercurrent to the aqueous butynediol solution, the solidaccumulating at the interface between the aqueous butynediol and thesolvent and the solid being removed from the column by taking off amixture of aqueous butynediol and solvent.

The aqueous butynediol solutions to be used according to the presentinvention have a butynediol content which can vary within wide limits.It is generally from 25 to 75% by weight, preferably from 40 to 60% byweight and particularly preferably from 45 to 55% by weight.

The solids content of the aqueous butynediol solution used is generallyfrom 1 to 1000 ppm, but in particular cases it can also be significantlygreater, eg. up to 5% by weight. The process of the present invention isparticularly advantageous for solids contents of from 2 to 500 ppm.

The removal of the solid is carried out in a column, which for thepurposes of the present invention means any apparatus in which thebutynediol solution can be passed from the top downward (downflow mode)in countercurrent to the solvent. On a small scale, glass columns as areused for chromatography are useful; on an industrial scale, suitablecolumns are, in particular, metal columns.

The butynediol solution to be purified is preferably introducedlaterally into the upper part of the column. To distribute the solutionuniformly in the column, customary distributor fittings such as tubedistributors have been found to be useful. The butynediol solution isgenerally introduced into the column in such a way that the flowvelocity in the column is from 0.1 to 0.4 m/min.

The butynediol solution is brought into contact with a solvent which hasa lower density than the butynediol solution and forms a second phasewith the latter. For this purpose, the solvents do not have to becompletely immiscible with the butynediol solution to be purified.Preference is given to those solvents which dissolve in the butynediolsolution to an extent of not more than 10% by weight, particularlypreferably not more than 3% by weight. Specific examples are ethers suchas methyl tert-butyl ether, ketones such as butyl methyl ketone,aromatic hydrocarbons such as benzene and aliphatic or cycloaliphatichydrocarbons such as cyclohexane.

The butynediol solution is preferably brought into contact with a C₄-C₁₈ -alcohol, particularly preferably a C₈ -C₁₀ -alcohol. Specificexamples are alkanols such as hexanol, heptanol, octanol, 2-ethylhexanoland decanol, where these alkanols can be straight-chain or branched andcan also be used in the form of mixtures of isomers.

The solvent is preferably introduced into the lower part of the column,with distributor fittings as in the case of the butynediol solutionhaving been found to be useful. The flow velocity of the solvent in thecolumn is generally selected so as to be from 0.15 to 0.4 m/min.

The volume ratio of the aqueous butynediol solution and the solvent isgenerally from 0.1:1 to 10:1, preferably from 0.5:1 to 3.5:1.

The column can be operated at from 0 to 100° C., preferably from 20 to50° C. To increase the contact area between the butynediol solution andthe solvent, preference is given to using packed columns. In a preferredembodiment, the column packing is arranged between the feed points forthe butynediol solution and the solvent. Suitable packing elements are,for example, ceramic rings, metal rings, Pall rings and Raschig rings.

The solvent rises toward the top of the column and the solid accumulatesat the interface between droplets of the aqueous butynediol solution andthe solvent. At the top of the column, a solvent phase is formed. At theinterface between the solvent phase at the top of the column and thebutynediol solution, there is a great accumulation of solid. For thisreason, a mixture of solid, butynediol and solvent is advantageouslytaken off at this interface and the solid is thus removed.

The solvent is advantageously discharged from the column via an overflowand returned to the process. Regulation of the amount of butynediolsolution flowing out at the bottom of the column enables the height ofthe interface between the butynediol solution and the solvent in thecolumn to be regulated.

In a preferred embodiment, the mixture containing the accumulated solidis taken off from the column and introduced into a separation vessel. Inthis vessel, the solvent and the butynediol solution separate, with thesolid remaining at the interface between the two liquids or sinking intothe lower butynediol phase. The solvent can be removed from theseparation vessel and returned to the column. The solid accumulates tosuch a great extent in the butynediol solution in the separation vesselthat it can be separated off by methods known per se, for examplefiltration, centrifugation or evaporation of the liquid, preferably bysedimentation. The butynediol solution which remains can be returned tothe column or processed further.

In the process of the present invention, it is generally sufficient toremove from 0.1 to 2% by weight of the original flow of the aqueousbutynediol solution to be purified from the column in the form of amixture of solvent, butynediol and solid.

The process of the present invention removes even low concentrations ofsolids from aqueous butynediol solutions in an economical manner.

EXAMPLES Examples 1-15

A column (15 cm diameter in Examples 1-11, 30 cm in Examples 12-15)containing a packing of 15 mm ceramic rings in Examples 1-11 and 2.5 cmmetal rings in Examples 12-15 was supplied continuously with aqueous,50% strength by weight butynediol solution at a point above the packing.n-Octanol in Examples 1-11 and also 14 and 15 and n-decanol in Examples12 and 13 was introduced into the column at a point below the packing.At the interface of alcohol and the aqueous butynediol solution, 0.5% byweight of the butynediol solution fed in was taken from the column inthe form of a mixture of alcohol, butynediol solution and solid. Thealcohol was discharged from the column via an overflow.

The table shows further parameters of the separation experiments.

                  TABLE                                                           ______________________________________                                                                       Solids                                                                        content of the                                       Feed rate                butynediol                                           of butyne-               solution Solid                                       diol     Feed rate                                                                              Tempera-                                                                             before/after                                                                           separated                             Ex-   solution of alcohol                                                                             ture   separation                                                                             off                                   ample [l/h]    [1/h]    [° C.]                                                                        [ppm]    [%]                                   ______________________________________                                        1     150      180      36     56/4     93                                    2     160      180      35     74/7     91                                    3     150      180      45     37/4     89                                    4     160      180      42      70/10   85                                    5     150      180      36     50/8     84                                    6     140      140      26      95/15   84                                    7     160      190      36     104/21   80                                    8     120      180      80      78/32   59                                    9     250      100      40      90/36   60                                    10     70      200      40      90/38   58                                    11     50      250      40      90/45   50                                    12    250      500      22     192/40   79                                    13    250      600      26      25/10   60                                    14    200      700      27     160/13   92                                    15    450      550      32     200/10   95                                    ______________________________________                                    

In all experiments, at least half, in some cases even more than 90% ofthe solid in the feed could be separated off.

Example 16

An aqueous, 50% strength by weight butynediol solution having a contentof 63 ppm was hydrogenated at 135° C. over a commercial catalyst at aweight hourly space velocity over the catalyst of 230 g of butynediol/1of catalyst.h (Experiment A). For comparison, a butynediol solutiondepleted in solids according to the present invention (residue solidscontent: 13 ppm) was hydrogenerated (Experiment B).

    ______________________________________                                                         Experiment                                                                            Experiment                                                            A       B                                                    ______________________________________                                        Residual butynediol content of the                                                               0.25      0.00                                             hydrogenation product [% by weight]                                           Residual butenediol content of the                                                               1.40      0.14                                             hydrogenation product [% by weight]                                           Content of by-products [% by weight]                                                             0.28      0.15                                             i) hydroxylbutyraldehyde                                                      ii) 2-(4-hydroxybutoxy)oxolane                                                                   0.37      0.02                                             ______________________________________                                    

Under the same conditions, the hydrogenation of the starting materialtreated according to the present invention was more complete than in thecase of the untreated material. Removal of the solid led to reducedformation of interfering by-products.

We claim:
 1. A process for separating solids from aqueous 1,4-butynediolsolutions, which comprises passing a solids-containing aqueousbutynediol solution in the downflow mode through a column and thusbringing it into contact with a solvent which has a lower density thanthe solids-containing butynediol solution and forms a second phase withthe latter, with the solvent rising in countercurrent to the aqueousbutynediol solution, the solid accumulating at the interface between theaqueous butynediol and the solvent and the solid being removed from thecolumn by taking off a mixture of aqueous butynediol and solvent.
 2. Aprocess as claimed in claim 1, wherein, in a column containing columnpacking, the solids-containing aqueous butynediol solution is introducedabove the column packing and the solvent is introduced below the columnpacking and the solid is removed from the column above the columnpacking.
 3. A process as claimed in claim 1, wherein the solvent used isoctanol.
 4. A process as claimed in claim 1, wherein aqueous butynediolsolutions having a solids content of from 1 to 1000 ppm are used.
 5. Aprocess as claimed in claim 1, wherein solids-containing aqueousbutynediol solutions having a butynediol content of from 40 to 60% byweight are used.
 6. A process as claimed in claim 1 wherein the mixtureremoved from the column, which is comprised of solids, the solvent andaqueous butynediol, is subsequently separated in a separation vesselinto a solids-containing aqueous butynediol solution and the solvent,wherein the solvent is optionally returned to the column and the solidsare separated by sedimentation from the aqueous butynediol solutionwhich is optionally returned to the column.